microstructure, corrosion, and fatigue properties of...

Journal of Surface Engineered Materials and Advanced Technology, 2011, 1, 101-106 doi:10.4236/jsemat.2011.13015 Published Online October 2011 (http://www.SciRP.org/journal/jsemat)

Copyright © 2011 SciRes. JSEMAT

Microstructure, Corrosion, and Fatigue Properties of Alumina-Titania Nanostructured Coatings

Ahmed Ibrahim1*, Abdel Salam Hamdy2

1Department of Mechanical Engineering, Farmingdale State College, Farmingdale, New York, USA; 2Max Planck Institute of Col-loids and Interfaces, Am Mühlenberg, Germany. Email: *[email protected] Received June 7th, 2011; revised August 5th, 2011; accepted August 14th, 2011.

ABSTRACT

Air Plasma spray process was used to deposit a conventional and nanostructured Al2O3-13 wt% TiO2 coatings on a stainless steel substrates. Morphology of the powder particles, microstructure and phase composition of the coatings were characterized by XRD and SEM. Potentiodynamic polarization tests and Electrochemical Impedance Spectro- scopy (EIS) were used to analyze the corrosion of the coated substrate in 3.5% NaCl solutions to determine the opti-mum conditions for corrosion protection. The fatigue strength and hardness of the coatings were investigated. The ex-perimental data indicated that the nanostructured coated samples exhibited higher hardness and fatigue strength com-pared to the conventional coated samples. On the other hand, the conventional coatings showed a better localized cor-rosion resistance than the nanostructured coatings. Keywords: Nanostructured Coatings, Alumina-Tiania, Fatigue Strength, Corrosion Resistance

1. Introduction

The plasma-sprayed Al2O3 coatings have been exten- sively used in many applications due to their thermal, chemical and mechanical stability. The Al2O3 phase is characterised by the highest chemical resistance among all oxides, good heat and electric insulations, high hardness and wear resistance, etc [1].

Plasma sprayed Al2O3–TiO2 (AT-13) coating is one of the most important coatings for many industrial applications [1-6]. It provides a dense and hard surface coat-ing which is resistant to abrasion, corrosion, cavitation, oxidation and erosion. AT-13 has been used for wear resistance, electrical insulation, thermal barrier applica-tions etc. several researchers reported that the Al2O3– TiO2 coating containing 13 wt.% of TiO2 showed the most excellent wear resistance among the Al2O3–TiO2

ones [2-6]. Nanostructured materials are one of the highest profile

classes of materials in science and engineering today, and will continue to be well into the future. Development of nanostructured ceramic coatings has become an impor-tant research area mainly due their interesting chemical, physical, and mechanical properties. For example, nanos-tructured AT-13 ceramic coatings show much higher wear resistance than conventional AT-13 coatings [3-7].

Fatigue and corrosion resistance are important proper-ties for many coatings selected for critical applications. Plasma-sprayed Al2O3 coatings are often used in corro-sion-resistant applications [8,9]. Because of their lamel-lar structure, ceramic coatings usually are characterized by a relatively high open porosity and incomplete bond-ing between lamellae, which are detrimental when the coatings have to perform in an aggressive environment. The porosity allows a path for electrolytes from the outer surface to the substrate [10,11].

It is widely recognized that thermal spray coatings can significantly influence the fatigue strength of coated co- mponents [12-14].

This paper presents the findings of a research on the microstructure, fatigue and corrosion behavior of thermally sprayed nanostructured and conventional AT-13 titania coatings.

2. Experimental procedure

2.1. Feedstock powders

Nanostructured (2613S) and conventional (ALO187) AT- 13 feedstock powders employed in this study obtained from Inframat Corp. (Farmington, CT, USA) & Praxair, Indiana, USA. The morphologies of both AT-13 powders are shown in Figure 1. The nanostructured was agglom-

Microstructure, Corrosion, and Fatigue Properties of Alumina-Titania Nanostructured Coatings 102

erated, spherical nanoparticles with high flowability and an average diameter of 30 µm. The conventional powder exhibited an angular and irregular morphology with size between 10 and 45 µm. Coatings were deposited using a Sulzer-Metco 9MB plasma torch under atmospheric conditions. Stainless steel cylindrical coupons were used as substrates. The typical spraying parameters for both conventional and nanostructured coatings are summa-rized in Table 1.

(a)

(b)

Figure 1. Morphologies of the AT-13 powders: (a) conven-tional; (b) nanostructured. Table 1. Summary of the plasma spraying parameters.

Parameters Conventional Coating NanoCoating

2.2. Characterization of Coatings

e compositions of t sprayed coa ere iffract D) using ps

(Philip 20). The c- d coating xamined by - electr py (SE

rdness mea were c on of the as-sp atings using Vickers

oatings were me-

used as the refer- acitively to a Pt wire to

ncies. EIS was per-

s 100 m (0.004 inches). The fa- conducted at room temperature

f

The phas he as tings wdetermined by X-ray dX-ray diffractometer ture of the as-sprayeeld emission scanning

The

ion (XR a Philimicrostrus APD 35

s was e a LEO fion cosurements

micros Monducted on

). microha

the cross secti rayed coIndentor. Microhardness values of the casured by digital hardness tester with load of 300 g on the cross-section of the polished samples.

2.3. Electrochemical Testing

2.3.1. Electrochemical Impedance EI

40 µm

S technique was used to evaluate the electrochemical behaviour of the coated samples in 3.5% NaCl solution open to air and at room temperature for up to three weeks. A three-electrode set-up was used with impedance spectra being recorded at the corrosion potential Ecorr. A saturated calomel electrode (SCE) wasence electrode. It was coupled capreduce the phase shift at high frequeformed between 0.01 Hz and 65 kHz frequency range using a frequency response analyzer, FRA, (Autolab PGSTAT30, Eco-Chemie, The Netherlands). The amplitude of the sinusoidal voltage signal was 10mV.

2.3.2. Polarization Measurements Linear polarization measurements were performed for the samples previously immersed for 30 minutes in 3.5% NaCl solution using Autolab PGSTAT30, Eco-Chemie, system. The scan rate was 0.05 mV/sec and the scan range was +/–20 mV with respect to the open circuit poten-tial. The exposed surface area was 4 cm2. All curves were normalized to 1 cm2 40 µm

2.4. Fatigue Testing

Rotating-beam fatigue testing was conducted on coated AISI low-carbon steel. The fatigue-testing machine is an RBF-200, rotating beam fatigue machine (Fatigue Dy- namics Inc., Walled Lake, MI). The test specimen used in the fatigue testing was a 12.7 mm (1/2-inch) hourglass bar prepared according to ASTM E466 [15]. The nominal coating thickness watigue experiments were

Torch Metco

Current

Voltage

Argon Flow Rate Argon

Pressure Hydrogen

Pressure Powder Carrier

Flow

Spray Distance

9MB

470 A

75 V

80 SCFH

100 PSI

50 PSI

15-18 SCFH

4.5 inch

9MB

500 A

70 V

80 SCFH

100 PSI

50 PSI

15-18 SCFH

4 inch

under a rotating beam fatigue and stress ratio of R = –1 configuration at a load frequency of 50 Hz. The surface of the specimen was prepared for coating by grit blasting with # 24 alumina, and no grinding was performed so as to not alter the surface roughness of the coatings. Fatigue life data generated in the fatigue tests were analyzed to determine the relationship between number of cycles to failure, N and probability of failure, P , for the samples tested.



uc- l2O3 after plasma spra-

e conven- lished that

-

o

al Figure 3(a)) has a a sprayed coatings,

3. Results and Discussion

3.1. Phase Composition of the Coatings

The XRD analysis of the AT-13 powders was confirmed in several previous studies [2-6] that both powders are prominently -Al2O3 Rutile phase of TiO2. The XRD analysis of the conventional and nanostructured coatings are shown in Figure 2(a) & (b). The XRD patterns of the coatings show that most of -alumina in the nanostrtured powder converted into -Aying process, which was similar to that in thtionally commercial powder. It is well estab

Al2O3 tends to be nucleated from the melt in preference to -Al2O3 due to the higher cooling rate [4]. It can be seen from Figure 2 that both nanostructured and conventional coatings mostly contained the -Al2O3 phase.

3.2. Microstructure of Coatings

The cross-sectional morphologies of the plasma sprayed coatings are shown in Figure 3. From the cross-sectional microstructures, it can be seen that both coatings consist

f the lamella built up from the molten droplets imping-ing on the substrate. In case of the nanostructured coating, the interface between the steel substrate and the coating appears much stronger than that of the conventioncoating. The “conventional” coating (layered microstructure, typical of plasm

Figure 2. X-ray diffraction of the AT-13 sprayed coatings: (a) conventional coating; (b) nanostructured coating.

which is the result of full melting (FM) of the ceramic feedstock powder and its solidification as “splats” on the substrate. The FM regions in the conventional coating consist of nanocrystalline -Al2O3 [1,17-20]. In all the coatings, some pores are observed, and splat boundaries are not clearly visible. A considerable amount (~16%) of partially melted regions (PM) is observed in the nanos-tructured coating (Figure 3(b)).

An SEM micrograph of the nanostructured coating is shown in Figure 3(b). This coating shows a bimodal microstructure composed of the two regions where nanopowders were fully (FM) and partially melted (PM). The partially melted region was formed when TiO2 was selectively melted because the temperature of nanopow-ders was not high enough during spray coating [2]. The partially melted (PM) rounded feature appears to consist of grains surrounded by a matrix phase, something simi-lar to microstructures of liquid-phase sintered materials [6].

(b)

Figure 3. Cross-sectional morphologies of the as-sprayed AT-13 coatings: (a) conventional; (b) nanostructured.

(a)

FM

20 µm

PM

20 µm



Figure 4 shows a high-magnification SEM image of the partially-melted (PM) region in the nanostructured coating. The partially-molten (PM) microstructural fea-tures consists mainly of submicron -Al2O3 fine equia- xed grains surrounded by a TiO2-rich amorphous phase.

3.3. Electrochemical Impedance Spectroscopy

EIS has been successfully applied to the study of corrosion systems for over thirty years and has been proven to be a powerful and accurate method for measuring corro-sion rates especially for coatings and thin films. An im-port vantage of EIS over other laboratory tech-niques is the possibility of using very small amplitude signals without significantly disturbing the properties being measured. To make an EIS measurement, a small amplitude signal is applied to a specimen over a range of frequencies.

The expression for impedance is composed of a real and an imaginary part. If the real part is plotted on the Z axis and the imaginary part on the Y axis of a chart, we get a “Nyquist plot”. However, Nyquist plots have one major shortcoming. When you look at any data point on the plot, cannot tell what frequency was used to re-cord that point. Therefore other impedance plots such as “Bode plots” are important to make a correct interpreta-tion. plots, the impedance is plotted with log freque the x-axis and both the absolute value of the impedance (|Z| = Z0) and phase-shift on the y-axis. Unlike the Nyquist plot, the Bode plot explicitly showfrequency information.

In this work, coated sam s were immersed in NaCl

ant ad

you

In Bode ncy on

s

plesolution for 21 days. Nyquist and Bode plots have been used to evaluate the corrosion resistance of the nanos-tructured and conventional sprayed coatings.

2

PM

µm

Figure 4. The partially melted region ‘PM’ consists of α- Al2O3 (dark) embedded in TiO2-rich amorphous phase.

The EIS results shown in Figure 5 and 6 as well as the visual inspection of the tested samples suggested that the conventional sprayed samples have a strong surface re-sistance to chloride ion corrosion after three weeks of immersion in NaCl solution. Figure 5 (Nyquist plot) showed that the surface resistance of the conventional coated samples is 2.0 × 104 Ω.cm2 which is almost 10 times of that obtained from the nanostructured coated samples, Al-Ti N, (2.5 × 103 Ω.cm2).

Figure 6 indicated that the pitting resistance of the co

ch contain m micro- and/or nano-pores affect negatively the the pitting corrosion resistance. Pitting corrosion can easily be done through the micro- or nano-pores in the nanocoatings. Therefore, the number of pitting areas increased sh- arply in the nanocoated samples compared with the conventional.

3.4. Linear Polarization Measurements

Polarization measurements (Figure 7) were performed for the conventional and naocoatings after 30 minutes of

FM

nventional coated samples was generally improved which was confirmed by the relaxation of the impedance spectra. It seems that the presence of partially melted regions (PM) in the nanostructured coating whi

any

Figure 5. Electrochemical impedance spectroscopy (nyquist Plots) of alumina_titania coated samples: Al-Ti C (Con-ventional), Al-Ti N (Nanostructured).

Figure 6. Electrochemical impedance spectroscopy (bode plots) of alumina-titania coated samples: Al-Ti C (Conven-tional), Al-Ti N (Nanostructured).



Figure 7. Polarization resistance measurments in NaCl so-lution. immersion in NaCl solution. Conventional coatings sam-ples showed a better polarization resistance than the nan-

he nanocoated samples.

3.5. Hardness of the Coatings

Vickers microhardness measurements were performed under a 300 gf load for 15 s on the cross-sections of the coatings. A total of 10 microhardness measurements were carried out for each coating. The mean microhardness of conventional coating is about 840, which is lower than the mean microhardness of 965 achieved by the nanostructured coating. The observed hardness differ- ence is believed to result partially from different coating

of TiO2 in these Al2O3/ TiO2 coatings affects the coating hardness. It is interesting to notice that the nanostructured coating contains partially melted regions (PM) contain many micro- or nano-pores and thus these regions should have a rela-tively low microhardness.

3.6. Results of the Fatigue Tests

Fatigue life data generated in the fatigue tests were analyzed to determine the relationship between stress level, S, number of cycles to failure, N and probability of failure, P

ocoated ones The corrosion potentioal of the conven-tional coated samples was shifted about 50 mV toward the passive direction compar d with te

microstructures and phase compositions, even though all these coatings have the same nominal chemical composition. Specifically, the distribution

f, for the samples tested. The failure probability [12], Pf, corresponding to the order number i is given by:

fP1n

(1)

Figure 8 shows the fatigue life (cycles) at 50% failure probability for the coated and uncoated specimens. The results indicate that the nanostructured alumina-titania coating exhibited higher fatigue lives compared to the conven

Figure 8. Fatigue life (cycles) for coated and uncoated specimens. fatigue strength of nanostructured coatings can be related to the crack propagation resistance of these coatings.

the crack propaga- tings is superior to

th

. Bansal et al. [20] have measured theconventional and the nanostye

microstructure of nanostructured coating is a bimodal in nature and it consists of regions of (FM) mixed with partially-molten (PM) microstructural features.

The presence of the partially melted (PM) region in the nanostructured coating act as a crack arrest and play a major rule in strengthening the crack propagation resistance.

The significant increase in the fatigue strength of the nanostructured coatings compared to the conventional coating is attributed to the improvemen

e presence of the partially melted regions (PM) in

Several investigators have shown thattion resistance of nanostructured coa

at of the conventional coatings [2,19,20]. It is important to notice that the partially melted (PM) regions in the coating act as a crack arrest as seen in Figure 3. Another important factor that can play a major role in increasing the fatigue resistance is the interfacial toughness

interfacial toughness of the ructured AT-13 plasma pra-

d coatings on steel substrates; the values were 22 and 45 J·m–2, respectively.

4. Conclusions

The microstructure of the conventional coating consists primarily of fully-molten (FM) regions. The

t in the crack propagation resistance resulted from the presence of the partially melted (PM) region.

Th

i

tional alumina-titania coating. The increase in the

the nanostructured coating which contain many micro- and nano-pores affect negatively the pitting corro-sion resistance. Therefore, conventional coatings showed a much better localized corrosion resistance than the nanocoatings.


Microstructure, Corrosion, and Fatigue Properties of Alumina-Titania Nanostructured Coatings


106

REFERENCES [1] J. Iwaszko, “Surface Remelting Treatment of Plasma-

Sprayed Al2O3 + 13 wt% TiO2 Coatings,” Surface and Coatings Technology, Vol. 201, No. 6, 2006, pp. 3443- 3451. doi:10.1016/j.surfcoat.2006.07.234

[2] E. H. Jordan, M. Gell and Y. H. Sohn, et al., “Fabrication and Evaluation of Plasma Sprayed Nanostructured Alu- mina-Titania Coatings with Superior Properties,” Ma- terials Science and Engineering A, Vol. 301, No. 1, 2001, pp. 80-89. doi:10.1016/S0921-5093(00)01382-4

[3] E. Song, J. Ahn, S. Lee and N. Kim, “Effects of Critical Plasma Spray Parameter and Spray Distance on Wear Resistance of Al2O3 – 8 wt% TiO2 Coating Plasma- Sprayed with Nanopowders,” Surface & Coatings Tech- nology, Vol. 202, No. 2, 2008, pp. 3625-3632.

[4] M. Gell, E. H. Jordan, Y. H. Sohn, D. Goberman, L. Shaw and T. D. Xiao, “Development and Implementation of Plasma Sprayed Nanostructured Ceramic Coatings,” Surface and Coating Technology, Vol. 146-147, No. 1, 2001, pp. 48-54. doi:10.1016/S0257-8972(01)01470-0

[5] A. Ibrahim, H. Salem and S. Sedky, “Excimer Laser Sur- face Treatment of Plasma Sprayed Al O + 13 wt% TiO2 3 2

Coatings,” Surface and Coating Technology, Vol. 203, No. 2, 2009, pp. 3579-3589.

doi:10.1016/j.surfcoat.2009.05.034

[6] D. Goberman, Y. H. Sohn, L. Shaw, E. Jordan and M. Gell, “Microstructure Development of Al2O3 – 13 wt% TiO2 Plasma Sprayed Coatings Derived from Nanocry- stalline Powders,” Acta Materialia, Vol. 50, No. 5, 2002, pp. 1141-1152. doi:10.1016/S1359-6454(01)00414-1

[7] H. Gleiter, “Nanostuctured Materials: Basic Concepts and Microstructure,” Acta Materialia, Vol. 48, No. 1, 2000, pp. 1-29. doi:10.1016/S1359-6454(99)00285-2

[8] M. Wang an of Powder Man- d L. Shaw, “Effects ufacturing Methods on Microstructure and WearPerfor- mance of Plasma Sprayed Alumina-Titania Coaings,” Sur-face and Coating Technology, Vol. 202, No. 1, 2007, pp. 34-44. doi:10.1016/j.surfcoat.2007.04.057

J. Zhang, Z. Wang, P. Lin, W. LU, Z. Zho[9] u and S. Jiang, “Effect of Sealing Treatment on the corrosion resistance of Plasma-Sprayed NiCrAl/Cr2O3 – 8 wt% TiO2 Coating,” Journal of Thermal Spray Technolog, Vol. 20, No. 3, 2010, pp. 508-513. doi:10.1007/s11666-010-9528-6

[10] S. Liscano, L. Gil and M. H. Staia, “Effect of Sealing Tr- eatment on the Corrosion Resistance of Thermal-Sprayed Ceramic Coatings,” Surface and Coatings Technology, Vol. 188-189, No. 1, 2004, pp. 135-139. doi:10.1016/j.surfcoat.2004.08.009

[11] J. Creus, H. Idrissi, H. Mazille, F. Sanchette and P.

Jacquot, “Corrosion Behaviour of Al/Ti Coating Elaborat- ed by Cathodic Arc PVStrate,” Thin Solid Films, Vol. 346

D Process onto Mild Steel Sub- , No. 1-2, 1999, pp.

C-Co,” Journal of Material Science, Vol. ience, Vol. 33, 12,

ome Plat-

e Fatigue and

150-154.

[12] A. Ibrahim and C. Berndt, “The Effect of High-Velocity Oxygen Fuel, Thermally Sprayed WC-Co on the High Cycle Fatigue (HCF) of Aluminum and Steel Thermally Sprayed with W33, No. 2, 1998, pp. 3095-3100.s Sc199

[13] A. Ibrahim and C. Berndt, “Fatigue and Deformations of HVOF Sprayed WC-Co Coatings vs. Hard Chring,” International Thermal Spray Conference, Vol. 1, No. 1, 2003, pp. 377-380.

[14] H. J. C. Voorwald, R. C. Souza, W. L. Pigatin and M. O. H. Cioffi, “Evaluation of WC-17Co and WC-10Co-4Cr Thermal Spray Coatings by HVOF on thCorrosion Strength of AISI 4340 Steel,” Surface and Coat- ings Technology, Vol. 190, No. 2, 2005, pp. 154-164. doi:10.1016/j.surfcoat.2004.08.181

[15] R. Ahmed and M. Hadfield, “Mechanisms of Fatigue Failure in Thermal Spray Coatings,” Journal of Thermal Spray Technology, Vol. 11, No. 3, 2002, pp. 333-349. doi:10.1361/105996302770348727

[16] Y. Wang, S. Jiang, M. Wang, S. Wang, T. D. Xiao and P. R. Strutt, “Abrasive Wear Characteristicsayed Nanostructured Alumina/Titan

of Plasma Spr- ia Coatings,” Wear,

terials & Design, Vol. 29, No. 1, 2008, pp.

Vol. 237, No. 1, 1999, pp. 176-185.

[17] R. S. Lima and B. R. Marple, “Process-Property-Perfor- mance Relationships for Titanium Dioxide Coatings En-gineered from Nanostructured and Conventional Pow- ders,” Ma1845-1855. doi:10.1016/j.matdes.2008.03.005

[18] R. S. Lima, S. Dimitrievska , M. N. Bureau, B. R. Marple, A. Petit, F. Mwale and J. Antoniou, “HVOF-Sprayed Nano TiO2-HA Coatings Exhibiting Enhanced Biocom- patibility,” Journal of Thermal Spray Technology, Vol. 19, No. 1-2, 2010, pp. 336-343. doi:10.1007/s11666-009-9442-y

[19] R.S. Lima and B. R. Marple, “Thermal Spray Coatings Engineered from Nanostructured Ceramic Powders for Structural, Thermal Barrier and Biomedical Applications: A Review,” Journal of Thermal Spray Technology, Vol. 16, No. 1, 2007, pp. 40-63. doi:10.1007/s11666-006-9010-7

[20] P. Bansal, N. P. Padture, A. Vasiliev, “Improved Interfa-cial Mechanical Properties of Al2O3 – 13 wt% Plasmasprayed Coatings Derived

TiO2 from Nanocrystalline

Powders,” Acta Materials, Vol. 51, No. 10, 2003, pp. 2959-2970. doi:10.1016/S1359-6454(03)00109-5

http://dx.doi.org/10.1016/j.surfcoat.2006.07.234


http://dx.doi.org/10.1016/S0921-5093(00)01382-4

http://dx.doi.org/10.1007/s11666-010-9528-6

http://dx.doi.org/10.1007/s11666-010-9528-6

http://dx.doi.org/10.1007/s11666-010-9528-6













http://dx.doi.org/10.1361/105996302770348727

microstructure, corrosion, and fatigue properties of...

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